How Metamagic Affects DCs and Ability Score Requirements

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There has been some confusion about how metamagic feats affect the DC of the spells they're applied to, and what influence the metamagic feats have on the ability score required to use the spell. Here's a summary.

    1) To cast a spell of level X, you need an appropriate ability score of 10+X (Int for Wiz, Wis for Clr/Drd/Pal/Rgr, Cha for Brd/Sor).

    2) Metamagic feats other than Heighten Spell do not change the actual level of the spell, just the level of the spell slot used.
    2a) Therefore, to cast a metamagicked spell (other than a heightened spell), you only need an ability score of 10+X (rather than 10+X+M, where M is the number of levels the metamagic feat modifies the original spell). So a Wiz9 with Int 11 could prepare and cast a quickened burning hands, since the spell's actual level is still 1 and therefore castable by the wizard despite her "low" Int.
    2b) Therefore, since the DC of the spell is based on the spell's actual level (not the level of it's spell slot used), the DC for a metamagicked spell is the same as the DC of the unmetamagicked spell: 10+X+A (where A is the relevant ability score mod, either of the caster or--if an item--the minimum needed to cast that spell). Therefore the above wizard's burning hands would have a DC of 11 (10+1+0), and a scroll of quickened burning hands would have that DC as well.

    3) Heighten Spell actually changes the level of the spell, making it more difficult to cast and more difficult to resist.
    3a) Therefore you use the modified level of the heightened spell to determine the minimum ability score needed to cast it.
    3b) Therefore you use the modified level of the heightened spell to determine the new DC.

If using any metamagic feat increased the DC of the spell, it greatly devalues the Heighten Spell feat (whose main purpose is to increase a spell's DC). In other words, you'd get all of the benefits of Heighten Spell whenever you used any other metamagic feat, so why ever use Heighten Spell?

If using a higher-level slot always meant you use the slot level instead of spell level to determine the DC, then that would completely negate the need for Heighten Spell, since a caster can always use a higher-level slot to cast a lower-level spell.